Some factors affecting the solubility of phosphates during the microbial decomposition of plant material

Abstract

Subterranean clover (Trifolium subterraneum L.) was decomposed under various conditions of incubation. Changes in the solubility of various phosphates and in pH were measured during the decomposition of this organic matter. Ferric phosphate and di- and tricalcium phosphates became more soluble when the clover was submerged and incubated under air or under nitrogen. Nauru rock phosphate and aluminium phosphate were not dissolved under these conditions. Ferric phosphate was not dissolved during incubation under well-aerated, moist conditions; it became more soluble as the degree of aeration of the mixture decreased and as the amount of clover, ferric phosphate, or water in the system increased. Ferric phosphate was dissolved by submerged clover in the presence of iron and aluminium oxides, but less phosphate and relatively more iron was in solution, owing to adsorption of the phosphate on the oxides. Bentonite did not decrease the solubility of ferric phosphate under similar conditions; however, calcium carbonate and kaolin caused a small decrease in solubility when added in large amounts. The pH of submerged clover incubated under nitrogen decreased from about 6.0 to between 5.0 and 4.5 and remained at this level; when incubated under air the pH was initially the same as under nitrogen but then rapidly increased to values as high as 8.5 During decomposition under well-aerated, moist conditions the pH steadily increased from 6.0 to 8.5.